This invention relates generally to reversible vapor compression refrigeration systems, commonly known as heat pumps, having variable speed compressors. More particularly, the invention relates to a method for detecting whether, after a system startup or mode change, the system flow reversing valve is positioned properly and for taking corrective action if it is not.
Reversible vapor compression refrigeration systems are widely used for space heating and cooling in a variety of applications. In the typical heat pump system, the refrigerant flow reversal required to change between the system cooling and heating modes is accomplished by a two position, four way or reversing valve. The reversing valve directs hot refrigerant from the compressor discharge to the outside heat exchanger during operation in the cooling mode and to the inside heat exchanger during operation in the heating mode. When the system is operating in the heating mode and the outside heat exchanger is functioning as the evaporator, frost can build up on the external surfaces of the heat exchanger, inhibiting heat transfer. Heat pump systems therefore usually have, in addition to a cooling and a heating mode, a defrost mode of operation. In the defrost mode, the refrigerant flow in the system is aligned as it is in the cooling mode, with hot refrigerant being directed to the outside heat exchanger, for a period sufficient to melt any frost buildup. There are a variety of defrost control schemes and methods in use.
In most heat pump systems used in buildings, the heat pump compressor is driven by an electric motor. Advances in motor control technology have led to the availability of variable speed motors, enabling their use to power fans and compressors in refrigeration systems such as heat pumps. Variable speed components allow for reduced system radiated noise during periods of light demand on the system, increased system efficiency and energy savings.
Most heat pump system flow reversing valves are solenoid actuated and pressure operated, depending on pressure differentials in the system to change from one position to the other. If these pressure differentials are reduced, as may be case when the system compressor is operating at some speed less than its maximum, the reversing valve may fail to operate or stick in some intermediate position between the heating mode position and the cooling mode position when commanded to shift positions in response to a system mode change. Improper operation of the reversing valve can seriously degrade system performance.
What is needed, therefore, is a means for detecting the position of the reversing valve in a heat pump system and for insuring that the valve is in the proper position.